The present invention relates to an image processing apparatus, and more particularly to an image processing apparatus which compares in level an analog image signal with a reference pattern signal recurring at predetermined periods, and produces a pulse-width modulated binary-coded image signal through the comparison.
2. Description of the Prior Art
An image processing apparatus, such as a laser printer, has been known as a means for forming a half-tone image. In the apparatus an input digital image signal is converted into an analog image signal, the analog image signal is compared with a periodic analog pattern signal, such as a triangle-wave signal, and a pulse-width modulated binary-coded image signal is produced through the comparison.
In producing a pulse-width modulated pulse signal through the comparison of the analog video data with the reference pattern to be compared, a phase relationship between the analog video data and the reference pattern frequently changes depending on a quantity of delay of each signal system. In this case, the output pulse width varies, and the pulse width nonlinearly varies according to the level of the analog video data signal. The resultant tone expression performance is unsatisfactory.
In another prior art (as disclosed in Unexamined Japanese Patent Publication Sho-62-181575), a bias level for the analog pattern signal is set so as to secure the maximum resolution and a linearity over the full scale in a manner that, by adjusting, for example, a quantity of bias applied to the triangle-wave, the maximum level of the analog video signal is set to be equal to the peak level of a triangle-wave signal, and the minimum level of the analog video signal is set to be equal to the bottom level of the triangle-wave signal. In the prior art, the characteristics of analog parts, such as resistors and capacitors, which make up a triangle-wave generator, vary due to temperature variation, for example, and consequently the bias level varies. Thus, it is difficult to set up an optimum bias level.